In recent times, it has become desirable and important to be able to characterize accurately the kinds of voiceband digital data modem signals being transmitted in a network. This information is important and useful for network planning and management. Such voiceband digital data modem signals are transmitted by a variety of digital data modem signal sources, e.g., modems, data sets and facsimile units.
As is well known in the art, digital data modems are employed to transmit and receive information in digital form (bits) over voiceband communication channels. Such digital data modems of interest employ passband pulse amplitude modulation techniques, for example, quadrature amplitude modulation (QAM), phase shift keying (PSK) and the like. Some known CCITT standard type digital data modems are CCITT V.29, CCITT V.32 and CCITT V.33. Other high speed digital data modems are also known in the art.
It is known that these types of digital data modems encode groups of one or more bits of supplied digital information into symbols at a symbol rate, i.e., baud. The symbol rate is typically equal to or less than the incoming bit rate for high speed applications. The symbols are represented as complex quantities, i.e., constellation points, selected from an alphabet of possible symbols for the particular digital data modem. The real and imaginary components of the complex quantities are modulated by inphase and quadrature, i.e., cosine and sine, respectively, carrier signals for transmission. Such arrangements are known in the art, see for example, Digital Communication, authored by E. A. Lee and D. G. Messerschmitt, Kluwer Academic Publishers, Norwell, Ma., 1988, Chapter 6, Sections 6.1-6.5, pages 146-206.
By way of example, a known CCITT V.29 digital data modem operating at 9600 b/sec encodes groups of four (4) incoming bits into one of sixteen possible symbols which are then transmitted at the symbol rate of 2400 baud.
Heretofore, attempts at characterizing the types of voiceband digital data modem signals being transmitted were simplistic and yielded inaccurate results. In one example, a signal is classified as being a voiceband digital data modem signal if an echo suppressor (canceler) disable tone is identified as being transmitted. Problems with such an arrangement are that all digital data modem signal sources do not transmit such disable tones, and the disable tone does not identify the specific type of digital data modem signal source or its transmission rate. Another problem with this prior arrangement is that the echo suppressor (canceler) disable tone typically is not transmitted in both directions of transmission. Therefore, both directions of transmission would have to be monitored in attempting to characterize the signals being transmitted in the network.
Another way of characterizing, i.e., identifying, such a digital data modem signal as to its source would be to simply use an array of digital data modem demodulators. Such an arrangement is undesirable because of the number of demodulators that would be required to detect the different types of digital data modem signals being transmitted. Additionally, as is known in the art, such demodulators must be on line at the on-set of the transmission of the digital data modem signal to insure synchronization and correct demodulation.
More recently, voiceband digital data modem signals have been classified as having a specific symbol rate and a specific modulation scheme, as described in co-pending U.S. patent application, Ser. Nos. 927,506 and 927,503, both filed on Nov. 6, 1986 now U.S. Pat. Nos. 4,815,136 and 4,815,137, respectively, both issued on Mar. 21, 1989. These classifier arrangements, however, also fail to identify the specific digital data modem source that is generating the voiceband data signal.